Preparation and Characterization of Silver Orthophosphate

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OPEN Preparation and characterization of silver orthophosphate photocatalytic coating on glass substrate Masahide Hagiri*, Kenichi Uchida, Mika Kamo Sasaki & Shofyah Sakinah

The photocatalytic activity of silver orthophosphate Ag3PO4 has been studied and shown to have a high photo-oxidation capability. However, there is few reported example of a simple method to

prepare Ag3PO4 coatings on various substrates. In this study a novel and simple method to immobilize A g3PO4 on the surface of glass substrates has been developed. A silver phosphate paste based on a polyelectrolyte solution was applied to a smooth glass surface. The resulting dried material was calcined to obtain a coating that remained on the glass substrate. The coating layer was characterized by X-ray difraction and energy dispersive X-ray spectrometry, and the optical band gap of the

material was determined. The results indicated that an Ag3PO4 coating responsive to visible light was successfully prepared. The coating, under visible light irradiation, has the ability to decompose methylene blue. Although the coating contained some elemental silver, this did not adversely afect the optical band gap or the photocatalytic ability.

Te development of sustainable energy utilization and environmental purifcation technologies is an important issue for ongoing development of society. In this context, research on photocatalysts is being actively conducted. Titanium dioxide TiO 2, a typical inorganic solid photocatalyst, is widely used due to its excellent photocatalytic activity and ability to produce hydrogen by water splitting1–5. However, the development of photocatalysts that utilize visible light, which accounts for 43% of sunlight, has been studied as an efective means of solar energy use3,4. Photocatalysts driven by visible light are also important in the utilization of indoor light as an energy source6. Recently, the photocatalytic activity of silver orthophosphate, Ag 3PO4, has been investigated, and the compound has been shown to have a high photo-oxidation capability 7–12. Photo-oxidative decomposition experiments using methylene blue revealed good photo-oxidative performance with a quantum yield of nearly 80%, which is tens of times more efcient than titanium dioxide or bismuth vanadate. In the photo-oxidative decomposition of water to generate oxygen, the performance of Ag3PO4 surpassed that of bismuth vanadate BiVO4 and tungsten 7 oxide WO 3 under visible light . Although silver phosphate cannot be used for the conversion of water to hydrogen due to its slightly low conduction band potential, it has shown considerable promise for oxygen generation, decomposition of organic matter, and in antifouling applications8. In addition, recent studies have revealed the antimicrobial properties of materials containing silver phosphate 13. For this reason, there have been numerous 7 reports on the synthesis and utilization of Ag3PO4, especially since the report of Yi et al. . Silver orthophosphate may be synthesized by precipitation 14–16, ion exchange 15,17, electrolysis7, and other methods 18,19, each of which has its own advantages. Recently, silver phosphate crystals of various shapes15,17–20, core–shell particles21,22, and composites, all of which provide high photocatalytic activity, have been studied 10,12,23–30. Te development of coating technologies for substrates is an important issue when using photocatalysts for surface antifouling applications 31. Tin-flm and thick-flm technologies for semiconductors are also important in the development and advancement of electronic devices 32,33. Te immobilization of titanium dioxide, a typical photocatalyst, on the surface of materials has been the subject of much research and has been widely used in practical applications 5,34–37. Tere are various ways for producing semiconductor flms, including the sol–gel method, vacuum deposition, sputtering, plasma CVD, and pulsed laser deposition. In the case of Ag 3PO4, it is difcult to use an approach

Department of Applied Chemistry and Biochemistry, Fukushima College, National Institute of Technology, Nagao 30, Kamiarakawa, Taira, Iwaki, Fukushima 970‑8034, Japan. *email: [email protected]

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such as the sol–gel method to synthesize a coating flm directly from a precursor solution on a solid surface, or a deposition method using plasma. 10 Noureen et al. investigated the antibacterial and photocatalytic properties of Ag3PO4/graphene oxide coated on cotton textiles. For these purposes, the coating has exhibited high performance. Furthermore, Xie et al.38 prepared titanium plates coated with polydopamine, graphene oxide, and Ag 3PO4 and successfully eliminated bioflm on the metal surface. Nevertheless, these materials were coatings in which crystals were electrostatically deposited on a portion of the substrate surface. An example of fabrication for an Ag 3PO4 coating that adheres to substrates is electrolytic deposition of silver plate, as reported by Yi et al.7. However, the substrate in this method is limited to a conductive medium such as silver plate. Te development of a simple method for the preparation of Ag3PO4 coatings on a variety of substrates may lead to more widespread use of Ag 3PO4 as a highly efcient photocatalyst. In particular, if a method can be established for forming flms on transparent materials such as glass, it will be possible to apply Ag3PO4 to optical devices and photoelectrodes, and this should contribute greatly to the development of optoelectronics using Ag3PO4. Tere are few reports for Ag3PO4 coating on transparent substrates. For example, Ma et al. successfully 39 40 immobilized an Ag/Ag2O/Ag3PO4/Bi2WO6 photocatalyst on a glass surface . Gunjakar et al. deposited Ag 3PO4 on the surface of ITO substrate by chemical bath deposition method. For the utilization of Ag 3PO4, it is desirable to discover more versatile coating methods. In this study, we have developed a novel and simple method to immobilize Ag 3PO4 as a coating on the surface of a glass substrate. Te approach is based on a simple method for preparing titanium dioxide coatings for dye-sensitized solar cells via a precursor paste, but the composition of the paste is even simpler in the present application. Tis article is based on a study frst reported in a short communication (in Japanese)41, to which substantial discussion and additions have been made. In this paper, the coating layer was obtained by frst preparing a paste, applying it to a glass surface, and then calcination. Te obtained coating layer was subjected to scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDX), and X-ray difraction (XRD) analyses. Also, the optical response of the samples was evaluated by measuring their difuse refection absorption spectra. Termogravimetry (TG) and diferential thermal analysis (DTA) were also performed to study the thermal reactivity of the dried paste. Materials and methods Materials. All the water used in the experiments was distilled once and then purifed by ion exchange. Sil- ver nitrate (Wako Pure Chemicals), disodium hydrogen phosphate (Kanto Chemical), carboxymethyl cellu- lose sodium salt (CMC-Na; Wako Pure Chemicals), and methylene blue (Wako Pure Chemicals) were used as received without purifcation. Te commercial silver phosphate (Sigma-Aldrich), silver (Sigma-Aldrich), barium sulfate (Wako Pure Chemicals) were also used without further purifcation. Silver orthophosphate was synthesized according to the method for producing silver phosphate fne particles reported by Khan et al.16. Specifcally, 100 mL each of 0.020 mol L−1 aqueous silver nitrate solution and 0.020 mol L −1 aqueous disodium hydrogen phosphate solution were dropped simultaneously into 200 mL of stirred pure water. To remove the supernatant and obtain fne crystals, the resulting colloidal solution of Ag3PO4 was centrifuged. Tis procedure was repeated at least three times, until no precipitation of silver chloride occurred, whereupon sodium chloride solution was added to the supernatant. Te resulting precipitate was dried under reduced pressure to obtain fne particles of Ag3PO4. Te resulting powder was yellow in color. To study the efect of the dropping rate on the product, the synthesis was attempted with two diferent dropping rates. Te XRD patterns and SEM images for the obtained samples were measured. Te XRD patterns for the samples synthesized at dropping rates of 0.020 cm3 s −1 and 0.33 cm3 s −1 are shown in Fig. 1. For comparison purposes, the difraction pattern for commercial silver phosphate (Aldrich, > 99%) is also shown. Te samples obtained using the above method showed prominent difraction peaks at 33.3° (210), 36.6° (211), 52.7° (222), 57.3° (321), and 55.0° (320)22. All of these major peaks were consistent with the commercial sample and assigned to the body-centered cubic structure of silver phosphate (JCPDS, card No. 6-505). Te results indicated that the synthetic products were Ag3PO4. A slight overlap of the halo pattern indicated the presence of small amounts of amorphous or low-crystallinity product. Te SEM images of the products obtained for each dropping rate are shown in Fig. 2. Te crystal shape is hexagonal prismatic, which is consistent with the crystal structure of silver phosphate. Te dropping rates of 0.33 cm3 s−1 and 0.020 cm3 s−1 yielded relatively uniform crystals of size ca. ~ 500 nm and ~ 1 µm, respectively. Te particle size is smaller for synthesis at the increased dropping rate. Te rapid dropping leads to rapid nucleation, resulting in an increase in crystal nuclei. Tis results in a smaller size of the yielded crystal. Subsequent studies were conducted using one of these microcrystals (dropping rate 0.33 cm3 s−1).

Silver orthophosphate coatings on glass substrate. CMC-Na was used as a dispersion stabilizer and thickener in the preparation of silver phosphate paste. Te Ag 3PO4 obtained above, pure water, and CMC-Na were mixed in a mass ratio of 1:1:0.020–0.050, and a paste was prepared by kneading for 10 min while maintain- ing constant humidity. Te CMC-Na and water were prepared in advance as an aqueous solution. A sufcient amount of the paste was then put onto a borosilicate glass substrate (18 × 18 mm) with both edges masked with 2.0 mm wide tape (3 M Scotch TM Mending Tape, 58 µm thickness 42). Te paste was spread using a glass squeegee with a smooth surface to form a uniform paste layer on the substrate surface. Te resulting paste layer was dried for 20 h at room temperature and under reduced pressure. Te dried coatings were then sintered in an electric furnace at 300–500 °C for 2 h.

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(a) u. (b) arb. y/

(210 ) (c) Intensit ) ) ) (211 ) (320 ) ) (321 ) (222 ) (421 (200 (310 (400 ) (420 ) (220 ) (332 (330 )

20 40 60 80 2 / degree

Figure 1. XRD patterns for samples synthesized at dropping rates of (a) 0.020 cm 3 s −1 and (b) 0.33 cm 3 s −1. For comparison, a difraction pattern measured for (c) commercial silver phosphate is also shown.

Dye decomposition experiment for evaluation of photocatalytic activity. To examine the photocatalytic activity of the coatings, methylene blue degradation experiments were conducted under blue LED light irradiation. A glass substrate with the Ag 3PO4 coating (as prepared in Sect. Silver orthophosphate coatings on glass substrate) was placed in 50 mL of a 20 mg L−1 methylene blue solution, placed in the dark, and stirred with a stirrer while aerating slowly. To observe only the decrease in concentration due to light irradiation of the sample, the adsorption of methylene blue on the sample was allowed to reach equilibrium. Afer reaching the adsorption equilibrium, the sample solution was irradiated with light from a blue LED light source. Te absorbance of the sample solution at 665 nm was then measured at regular intervals. From the change in absorbance, the time course of the methylene blue concentration was measured. Te blue LED used had a central emission wavelength of 462 nm and an irradiance density of 1.0 mW cm−2 (calculated as monochromatic light).

Apparatus. Scanning electron microscopy was performed using a Hitachi S-3400 scanning electron micro- scope in backscattered electron mode under high vacuum. Elemental analysis in the scanning electron micro- scope was performed using an Ametek EDAX energy dispersive X-ray analyzer. Crystallographic analysis was performed using a Shimadzu XRD-6000 X-ray difractometer with CuKα radiation (λ = 0.15418 nm). To investigate the optical response of the samples by difuse refectance spectral measurements, a JASCO V-560 UV– Vis-NIR absorption spectrophotometer equipped with a JASCO ISV-469 type integrating sphere was used. A solid sample cell with a quartz window was used for the measurements. Barium sulfate powder was used as the white light reference standard. A Shimadzu Biospec-1600 UV–visible absorption spectrophotometer was used for determination of the methylene blue concentration in aqueous solution. Te concentration was determined using a calibration curve prepared from the absorbance of methylene blue standard solutions at 665 nm. For thermal analysis, a Shimadzu DTG-60H thermal analyzer was used to record the diferential heat and thermo- gravimetric changes with increasing temperature. Results and discussion Formation of silver phosphate coatings on borosilicate glass substrate and characterization. First, the silver phosphate and CMC-Na solution were well mixed to prepare a paste. Te paste was applied to a borosilicate glass plate and squeegeed to spread the paste uniformly. Silver phosphate paste with a mass ratio of silver phosphate, pure water, and CMC-Na of 1.0:1.0:0.010–0.050 was spread out and dried. For the range of 1.0:1.0:0.020–0.050, well-dried coatings adhered to the glass without cracks or voids were obtained. Te coatings retained their structure on the glass surface even afer sintering at 300 °C or 500 °C and washing the resulting coatings with distilled water. Afer sintering, there were no cracks in the coatings for specimens in the composition range 1.0:1.0:0.025–0.030, while cracks were observed for the specimens outside this range. It was also thought that it was necessary to study the process at a higher temperature. However, given that the

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Figure 2. SEM images of microcrystals obtained for dropping rates of (a) 0.020 cm 3 s −1 and (b) 0.33 cm 3 s −1.

melting points of orthophosphate Ag3PO4, pyrophosphate Ag4P2O7, and metaphosphate AgPO3 were reported to be 849 °C43, 585 °C44, and 482 °C45, respectively, and since a lower-temperature deposition process was also desirable from an energy standpoint, we proceeded with experiments in this temperature range. Te thickness of the coating obtained by sintering the paste-casted substrate was determined by cross-sectional observation by SEM and the thickness was 20 ± 3 µm (n = 4). Te thickness of the coating was smaller than the spacer thickness (3 M Scotch™ Mending Tape, 58 µm) used in paste application, because of shrinkage during drying and sintering. Te results of characterization of the coating prepared using silver phosphate paste with a mass ratio of 1.0:1.0:0.030 (Ag3PO4:H2O:CMC-Na) are shown below. SEM images of the surface of the substrate coated with the paste and calcined at 300 °C (a) and at 500 °C (b) are shown in Fig. 3. In the case of sintering at 300 °C (a), some particles were observed to remain, but the boundary between the particles was barely discernable due to coalescence. When the coating was sintered at 500 °C (b), the existence of particles did not be confrmed. However, a structure with a large number of pores in the micrometer to sub- micrometer size range was observed in both coatings afer calcination. An EDX analysis of the coating during SEM revealed silver, phosphorus, oxygen, and a trace of sodium. Te elemental abundance ratio for silver and phosphorus was approximately Ag/P = 3, which is consistent with the stoichiometry of silver phosphate. Te XRD patterns for the silver phosphate coating obtained by calcination are summarized in Fig. 4, the patterns for calcination at 300 °C and 500 °C being shown in Fig. 4b, c, respectively. For comparison purposes, the XRD pattern for the dried paste before calcination is also presented (Fig. 4a). Te difraction intensities are normalized with respect to the intensity of the largest difraction peak.

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Figure 3. SEM images of surface of substrate coated with silver phosphate via calcination at (a) 300 °C and (b) 500 °C.

In the XRD pattern for the dried paste before calcination, only the peak for silver phosphate was identifed. However, peaks due to elemental silver (JCPDS, card no. 04-0783), indicated by the inverted triangle marks, were also observed for the samples calcined at 300 °C and 500 °C. Tis is attributed to the thermal reduction of silver phosphate or some silver compound generated during calcination. Te only silver source is silver phosphate, however since CMC-Na is added, complexes of silver and CMC may also be involved. In the sample calcined at 300 °C, there is a peak at around 32° that is not seen for the other samples. Te difraction peaks were compared with the XRD data of CMC-Na, several polymorphs of carbon, and several silver salts. And the most consistent with silver (I) oxide 46. Since silver oxide decomposes into oxygen and silver at about 300 °C, the fact that this peak disappeared upon calcination at 500 °C also supports this identifcation 47. Termogravimetry (TG, dashed line) and diferential thermal analysis (DTA, solid line) curves measured for dried silver phosphate paste are presented in Fig. 5. Te measurements were performed on a paste that had been dried on a glass substrate and was then scraped of with a metal scraper. Te curves indicate that a decrease in weight up to a heating temperature of around 350 °C occurred, and the weight increased slightly above 350 °C. Tis mass loss was attributed to the removal of water and the CMC-Na contained in the dry paste as a result of the heating. Dhanabal et al.48 reported the results of a TG–DTA analysis of silver phosphate synthesized by precipitation and hydrothermal treatment. Tey reported that a small endothermic peak corresponding to the partial melting of Ag3PO4 was observed at 531 °C or 526 °C depending on the synthesis conditions. In the TG–DTA analysis in the present study, an endothermic peak was also observed at 523 °C. In addition to this, an endothermic peak at 355 °C was noted. Tis latter peak may also correspond to partial melting of the phosphate. Another endothermic peak was detected at a lower temperature, even though

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(a) u. (b) arb.

Intensit y/ (c)

20 40 60 80 2 /degree

Figure 4. XRD patterns for silver phosphate coating obtained by calcination at (b) 300 °C and (c) 500 °C. An XRD pattern for (a) the dried paste before calcination is also shown.

0.0 8.0 DT A/ 4.0

/% -2.0 V TG 0.0

-4.0 -4.0 100 200 300 400500 Temperature/°C

Figure 5. Results of diferential thermal analysis (solid line) and thermogravimetry (dashed line) of a silver phosphate paste.

the particle size of the phosphate was almost the same as that synthesized by Dhanabal et al.48. Tis is due to coordination of the carboxyl groups of CMC-Na with silver. However, the infuence of the coexistence of sodium ions cannot be ruled out. Next, an experiment was conducted to investigate the cause of silver formation during heating of the paste. Te XRD patterns were measured for samples of silver phosphate paste with a mass ratio of 1.0:1.0:0.030, which were calcined at 200–500 °C, and ground in an agate mortar. Te XRD patterns for raw silver phosphate calcined at 200–500 °C were also measured and compared. Te results are shown in Fig. 6.

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(a)

u. (b) arb. y/ (c) Intensit (d)

30 35 40 45 50 2 /degree

Figure 6. XRD patterns for raw silver phosphate calcined at (a) 300 °C, (b) 500 °C and silver phosphate paste calcined at (c) 300 °C, (d) 500 °C. Here, the composition ratio for silver phosphate paste is 1.0:1.0:0.030 (Ag 3PO4:H2O:CMC-Na). Te dashed vertical line represents the difraction angle for elemental silver.

Te strongest peak for elemental silver appears near 38°. Te peak did not appear in the difraction pattern if silver phosphate was only calcined at 300 °C, and it was only slightly observed at 500 °C. In contrast, in the presence of CMC-Na, the peak of 38° was observed in both 300 °C and 500 °C. Te intensity of the difraction peak around 38° due to elemental silver is defned as IS, and the intensity of the difraction peak around 33° due to silver phosphate is defned as IP. Te ratio IS/IP was determined and plotted against temperature as shown in Fig. 7. Te increase in the IS/IP ratio at high temperatures is signifcantly larger with the presence of CMC-Na. Tis result clearly shows that CMC-Na plays a positive role for silver formation, although silver formation by direct heating cannot be ruled out. Te difuse refectance absorption spectra of the samples were measured to characterize the photoresponsive properties of the coatings and to determine the optical band gap. Te specifc refectance R = RS/RRef was obtained from the refectance RS for each sample and the refectance RRef for the standard material, barium sulfate. Te KM function F(R) was calculated from the R value using the Kubelka–Munk Eq. (1):49 2 F(R) = K/S = (1 − R) /2R (1) where K is the absorption coefcient and S is the scattering coefcient. Te wavelength dependence of F(R) is shown in Fig. 8 for the coatings obtained by calcination at 300 °C (dashed line) and 500 °C (solid line). Both coatings showed a wide range of absorption over the UV to visible region. Te absorption in the visible region extended to nearly 500 nm. Tis fnding is consistent with the appearance of the samples. Terefore, the coatings obtained in this study are confrmed to be responsive to visible light and are expected to exhibit photocatalytic activity in this wavelength range. Te band gaps of the coatings obtained by calcination at 300 °C and 500 °C were estimated using a Tauc plot 50, which is a plot of (αhν)n against the photon energy hν, where α is the absorption coefcient of the material. For band-gap transitions of silver phosphate, n = 1/2 because they are indirectly allowed transitions 51,52. Although it is difcult to measure the absolute value of the absorption coefcient K by conventional spectral refectance measurements, it is assumed that the scattering coefcient S is constant and α = F(R). Te Tauc plot obtained in this study is shown in Fig. 9. For the coating calcined at 500 °C, a clear edge appeared around 2.5 eV. From the intersection of tangental lines (red dashed line) drawn on the absorption edge and the baseline, the band gap energy was estimated to be 7 53 2.36 eV. Yi et al. reported the band gap of Ag 3PO4 to be 2.4–2.5 eV. Opoku et al. estimated the band structure of bulk Ag 3PO4 by frst-principles calculations and reported a value of 2.43 eV as the indirect band gap. Te estimated band gap of 2.36 eV results in good agreement with these values. However, the coating calcined at 300 °C did not show a clear edge in the visible region. Tis is due to the formation of carbon and silver compounds derived from CMC-Na during calcination at low temperatures, which raise the baseline. Terefore, the band gap of this sample did not estimated because an absorption edge of Ag3PO4 did not be identifed.

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1.0

0.8

0.6 P I / s I 0.4

0.2

0.0 0200 400 Temperature/°C

Figure 7. Plot showing the correlation between IS/IP and the calcination temperature. Here, the intensity IS of the difraction peak near 38° is for metallic silver, and the intensity IP of the difraction peak near 33° is for silver orthophosphate. Open circles indicate data for raw silver phosphate, and closed circles indicate data for the paste (mixture with CMC-Na). u. arb. )/ R ( F

300 400 500600 700800 Wavelength /nm

Figure 8. Wavelength dependence of F(R) for coatings obtained by calcination at 300 °C (dashed line) and 500 °C (solid line).

Te results of the present experiments confrm the photocatalytic activity of silver phosphate as outlined below. Te results clearly demonstrate that the present coating exhibits light absorption in the visible region and promising photocatalytic activity. Clearly, a silver phosphate coating can be prepared from a simple paste consisting of silver phosphate and CMC-Na from these results, Te coating has a large number of micropores on its surface and is responsive in the

Scientifc Reports | (2021) 11:13968 | https://doi.org/10.1038/s41598-021-93352-z 8 Vol:.(1234567890) www.nature.com/scientificreports/ . .u rb /a 2 1/ ) h (

2.36 eV

1.52.0 2.53.0 3.54.0 Photon energy /eV

Figure 9. Dependence of (αhν)1/2 on photon energy hν (Tauc plot) for samples sintered at 300 °C (dashed line) and 500 °C (solid line).

visible light region. Although the precursor paste used in this study contained only a polyelectrolyte as a disper- sant and no inorganic ultrafne particles as a sintering additive, an excellent coating was successfully prepared. Tis is due to the low melting point of Ag3PO4, which readily self-sintered and recrystallized upon heating, resulting in the disappearance of the particle interfaces. Te coexistence of CMC-Na had the efect of enhancing the ease of this change. We found that the coating contained a small amount of metallic silver due to the coexistence of CMC-Na. For a semiconductor deposition process, it is desirable to have a single-phase composition. However, a single phase is not always the best from the perspective of photocatalytic activity. For example, Zhu et al.21 prepared an Ag3PO4 photocatalyst contained metallic silver, and reported that its photocatalytic ability was higher 20 than that of Ag3PO4 alone. Zhang et al. also reported that the coexistence of graphite carbon nitride enhances the photocatalytic performance of Ag3PO4. Tere have been many reports on the enhancement of photocatalytic activity by plasmonic efects in combination with silver. Dong et al.54 succeeded in improving the performance of an Ag3PO4 photocatalyst by a simple sintering method. Te elemental silver produced by calcination is benefcial for the rapid transfer of photoexcited electrons of Ag3PO4 and inhibits the photocorrosion of Ag 3PO4, thereby improving the stability of the photocatalyst. It is also assumed that elemental silver is produced during sinter- + ing because Ag in the Ag 3PO4 semiconductor is reduced by thermally excited electrons. Te aforementioned researchers reported the formation of elemental silver by calcination of pure silver phosphate. Tis may seem to contradict the results shown in Fig. 6. However, even in the report by Dong et al., no peaks for elemental silver appeared in the XRD pattern due to calcination of silver phosphate, and our results are consistent with this. Te coexistence of CMC-Na salt facilitated the formation of silver, which is attributed to the efect of carboxy groups coordinating with ionic silver to lower its melting point. Te phenomenon that CMC lowers the melting point of silver-related substances and facilitates the formation of silver has been reported by Miyama et al.55.

Dye decomposition experiment for evaluation of photocatalytic activity. Te decomposition of organic dyes was attempted by irradiating the coated sample with blue LED light in a methylene blue solution. By examining the degradability of organic dyes, the photocatalytic activity during visible light illumination can be confrmed. One glass substrate with a coating was placed in 50 mL of 20 mg L−1 methylene blue aqueous solution and allowed to reach adsorption equilibrium in the dark. Afer that, the methylene blue was irradiated with a blue LED (central wavelength 462 nm, 1.0 mW cm−2) and the absorbance of the aqueous solution was measured at regular intervals to confrm whether the methylene blue had decomposed. Te results are shown in Fig. 10. Here, C0 is the initial concentration and Ct is the concentration at time t. Obviously, only the Ag 3PO4 coating caused a signifcant decrease in methylene blue concentration under visible light irradiation. Tis suggests that the methylene blue was decolorized or decomposed by the photocatalytic reaction using the coating under visible light. Tis clearly indicates that the coating obtained in this study pos- sesses photocatalytic activity under visible light irradiation. Fitting the data to a single exponential decay curve, the rate constant and time constant for decomposition by the coating sintered at 500 °C were estimated to be 0.0146 min−1 (2.45 × 10–4 s−1) and 68.2 min (4.09 × 103 s), respectively. In contrast, the coating at 300 °C exhibited

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1.0

0.8 0

C 0.6 / t C 0.4 Blank 0.2 Ag3PO4 /300 °C Ag3PO4 /500 °C

0.0 0100 200300 Time /min

Figure 10. Time course of specifc concentration Ct/C0 of methylene blue under blue LED illumination for (1) substrate only, (2) silver phosphate coated substrate (calcined at 300 °C), and (3) silver phosphate coated substrate (calcined at 500 °C). Here, C0 is the initial concentration of methylene blue and Ct is the concentration at time t.

slower decomposition than that at 500 °C. Te coating at 300 °C contains impurities and defects, as discussed above. Te impurities and defects promoted carrier recombination, resulting in low photocatalytic efciency56. 40 Gunjakar et al. prepared Ag 3PO4-deposited ITO substrates by chemical bath deposition route and studied 2 −2 the degradation of methylene blue by the substrates. A 2 cm substrate with 0.9 mg cm of Ag3PO4 deposited per area was immersed in 3 mL of methylene blue solution, and irradiated with visible light (> 420 nm). As a result, the methylene blue decomposed in approximately 1 h. Although the initial concentration of methylene blue did not found in this literature, the frst order decomposition rate constant is estimated to be 0.017–0.050 min−1 (2.8–8.3 × 10–4 s−1). Te result was comparable to the fndings of present study. Te comparison with a previous report above shows that the coating has sufcient degradation efciency, nevertheless the efect of silver coexistence in the coating is not clear. Terefore, a comparison with previous 57 reports on the composite Ag/Ag3PO4 was conducted. Piccirillo et al. studied the degradation of methylene blue by Ag/Ag3PO4 sample prepared by ion exchange of calcium phosphate with silver ion. As a result, the frst- order decomposition rate constant was determined to be 9.8–12.8 × 102 h −1 (0.27–0.36 s−1). In this experiment, the initial concentration of methylene blue was 5 mg L−1, the concentration of photocatalyst in the slurry was 0.25 g L −1, and a visible lamp of 50 W m−2 (5 mW cm −2) was used as the light source. Liu et al.58 synthesized Ag/Ag3PO4 by solid phase reaction and subsequent photoirradiation, and also attempted to degrade methylene blue. Te frst-order decomposition rate constant was determined to be 0.376–0.713 min−1 (0.0063–0.012 s−1). In the report, the initial concentration of methylene blue was 20 mg L−1, the concentration of photocatalyst in the slurry was 3 g L−1, and the light source was a 450 W Xe lamp with a cutof for light below 420 nm. Te rate constants obtained in our experiments are about one order of magnitude smaller than those of Liu et al. Although the initial concentration of methylene blue in our experiments was about the same as 20 mg L−1, the amount of photocatalyst on the glass substrate was small (ca. 4.5 mg cm−2, 2.9 cm2), and we used a weak LED light source with long wavelength (462 nm, 1.0 mW cm−2). Tese results indicate that the decomposition reaction is not extremely slow, but rather comparable. From this, we conclude that the coexistence of silver did not adversely afect the decomposition ability of methylene blue. Meanwhile, we found also no evidence that the coexistence of silver accelerated the degradation. It is necessary to conduct experiments under the same conditions using samples with controlled silver production to reveal the efect of silver coexistence. Since much study is needed to control the amount of silver produced, further research is now in progress. Te reusability of the catalyst is one of the most important parameters of its application as heterogeneous photocatalyst. For this reason, the reusability of the prepared Ag3PO4 coating as a photocatalyst was examined. Afer decomposing methylene blue under visible light irradiation as in Fig. 10, the coating was placed in a new methylene blue solution and exposed to light again. Te time course of the methylene blue concentration is shown in Fig. 11. In four repeated trials, most of the methylene blue was photocatalytically degraded. Tis indicates that the coating has a high photocatalytic capacity and maintains it during repeated trials. Katsumata et al.59 evaluated the stability of Ag 3PO4 photocatalyst by repeated experiments of bisphenol-A degradation. Te results showed

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Run1 Run2 Run3 Run4 1.0

0.8 0

C 0.6 / t C 0.4

0.2

0.0 0 180 360 540 720 Time /min

Figure 11. Time course of specifc concentration Ct/C0 of methylene blue under blue LED illumination for silver phosphate coated substrate (calcined at 500 °C) during the recycling test. Here, C0 is the initial concentration of methylene blue and Ct is the concentration at time t.

that the photocatalytic activity of Ag 3PO4 was efectively maintained even afer fve recycling runs. Our results are in agreement with this report. Te results of this experiment indicate that the prepared Ag 3PO4 coating can be reused as a photocatalyst. Conclusion In this study, a novel and simple method for immobilizing Ag 3PO4 on the surface of glass substrates was developed. A paste consisting of Ag 3PO4, water, and polyelectrolyte was applied to the glass surface, dried, and then calcined to obtain a coating that remained on the glass substrate. Te coating layer was characterized by XRD and EDX, and the main crystal phase of the coating was confrmed to be Ag3PO4. Ten, the visible light responsivity of the coating was evaluated by difuse refectance spectral measurement and decomposition of organic dyes under visible light irradiation. Te results showed that the coating was responsive to visible light and showed degradation activity for organic dyes. Te coating contains elemental silver generated during the sintering process, and this origin was also examined. For this purpose, the efects of the presence of CMC-Na and calcination temperature on silver formation were studied, and the results clearly show that CMC-Na plays an active role in the formation of silver, although silver formation by direct heating also occurs.

Received: 10 March 2021; Accepted: 21 June 2021

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